Industrial control systems are an important and necessary element in automating machinery and processes in industrial applications worldwide. Industrial control systems implement mechanical, hydraulic, pneumatic, and electrical controls as well as provide a user with a means to interface with these controls. The types of user interfaces can vary from a simple pushbutton to complex, computerized touch screens. Regardless of the type of user interface, each user interface must be presented to the user in an enclosure that is suitable to the environment in which the interface operates. As such, significant engineering work goes into designing enclosures that are not only appropriate to house a user interface but are also cost-effective to manufacture and maintain. Another important consideration of enclosure design is how to fit as many controls as possible into the smallest footprint while providing desired operability.
The many and varied types of industrial environments present different types of hazards and concerns. A major safety concern in dust laden industrial plants is the occurrence of fires and explosions. A source of energy is all that is needed to ignite an explosion when flammable gases or combustible dusts are mixed in the proper proportions with air. That source of energy can come from any number of sources within a plant environment however one area of particular concern is the plant's electrical installation which might include electrical components such as switches, circuit breakers, motor starter, pushbutton stations, or plugs and receptacles.
As such, the National Electric Code (NEC) has worked to establish a classification system to classify hazardous locations and to define standards for the types of electrical equipment that may be allowed into the hazardous location. The classes define the type of explosive or ignitable substances which are present in the atmosphere. Class I locations, those locations of particularly relevance to the present disclosure, are those in which flammable vapors and gases may be present. Class I is further subdivided into two divisions. Division 1 is a location in which ignitable concentrations of hazards exists under normal operation conditions and/or where hazard is caused by frequent maintenance or repair work or frequent equipment failure. Division 2 is a location in which ignitable concentrations of hazards are handled, processed or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of such containers or systems.
With regard to the types of electrical equipment that may be allowed in a Class I, Division 1 or Division 2 location, the NEC noted that any type of conventional relay, contact, or switch that has an arcing contact must be enclosed in an explosion proof housing. The NEC has defined an Explosionproof Apparatus as an apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or a vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.
Thus, an explosion proof enclosure must prevent the ignition of an explosive gas or vapor that may surround it. In other words, an explosion inside the enclosure must be prevented from starting a larger explosion outside the enclosure. An explosion proof enclosure must further be of adequate strength and be “flame-tight.” The term “flame-tight” does not imply that the enclosure is hermetically sealed but rather that the joints or flanges are held within narrow tolerances. These carefully machined joints cool the hot gases resulting from an internal explosion so that by the time they reach the outside hazardous atmosphere they are not hot enough to cause ignition.
Alternatively, intrinsically safe equipment may be used in a Class I, Division 1 or Division 2 location, wherein intrinsically safe equipment and wiring are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture intis most easily ignited concentration.
The constraints around the types of enclosures that may be placed within a Class I, Division 1 or Division 2 location, must be addressed by the engineers designing and building the enclosure. Intrinsically safe barriers and complex flame paths require significant design and construction efforts at significant cost. Consequently, electrical control designs have focused on simplified push-buttons and simplified selector switches within explosion proof enclosures.